DE2459612A1 - CAPACITIVE PRESSURE TRANSDUCER - Google Patents

Description

The invention relates to a pressure transducer in which a pressure-responsive capacitor and a reference capacitor are used finds.

To convert the changes in capacity of a pressure-sensitive A reference capacitor is used in many cases to convert the capacitor of a pressure transducer into an electrical output signal. The reference capacitor and the pressure-sensitive capacitor can be connected in branches of a bridge circuit be, or the one capacitor can be connected to the input of an amplifier be connected, while the other capacitor in a running from the output to the input of the amplifier feedback line is switched so that the output signal is proportional to the ratio of the two capacitances and is a function the pressure to which the pressure transducer is subjected.

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In the previously known capacitive pressure transducers, there is under among other things, the problem that an incorrect pressure display is caused by temperature changes. By temperature changes the distance between the plates of the capacitors is changed, which is due to the thermal expansion. Will the Pressure transducers used in an airplane so it can experience large temperature differences be exposed. The size of the false indication mainly depends on the dimensions used for the pressure transducer: materials, structural details and the temperature range in which the pressure transducer operates. It occurs when bringing about the desired degree of stability of the capacitor presents difficulties, especially when the pressure transducer is exposed to very strong temperature changes.

The invention is therefore intended to create a capacitive pressure transducer that can be exposed to very different temperatures without affecting the accuracy of the generated by it Pressure signal is adversely affected.

According to the invention, the pressure transducer comprises a hollow housing with two opposing walls that are spaced apart from one another insulating material, which are connected to one another at their edges by insulating material, with those of the Edges of remote areas of the walls can be deflected by pressure changes, while those at the edge areas of the walls are essentially not deflectable. The electrodes of the pressure responsive capacitor are on the opposing surfaces of the deflectable areas

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; arranged while the electrodes of the reference capacitor on the opposing surfaces of the non-deflectable Areas are arranged.

In the following, an embodiment of the invention is referred to on, the accompanying drawing explains in more detail. It shows:

1 shows a plan view of a pressure sensor according to the invention;

Fig. 2 is a vertical section through the invention Pressure sensor; and

Fig. 3 shows a circuit which shows the ratio of the two Generates an output signal proportional to the pressure exerted on the pressure sensor, which is proportional to the capacitance.

The one in FIGS. 1 and 2 illustrated pressure transducers according to the invention A housing 1 made of dielectric material, for example made of glass or quartz. The housing has two shell-shaped parts 3, 5, which are connected to one another at their edges by a suitable seal 6. They delimit a chamber lying between them . 7, which can be evacuated in a known manner. On the opposing inner surfaces of the deflectable sections of the bowl-shaped parts 3 and 5 are conductive ones that measure the pressure

; j

'Plates 9 and 11 arranged, whether by vacuum vapor deposition or; Sputtering can be made. These form a condenser, ■ which corresponds to the pressure exerted on the housing 1 from the outside. -4-

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speaking signal generated. On the opposing inner surfaces of the shell-shaped parts 3 and 5 are also at the seal 6 conductive plates 19, 21 are arranged. These form a reference capacitor which generates a reference signal. The plates 9 and 11 or 19 and 21 of the capacitors are connected to electrical conductors 13 and 15 or 23 and 24. By these are the capacitors, as shown in FIG. 3, connected to an electrical circuit.

The deflection of the shell shape parts 3 and 5 at their edges can be calculated using equation (1) below. This deflection is so small that the reference capacitor can be provided on the shell-shaped parts 3, 5 without that noticeable changes in capacitance occur across the pressure range to be measured. Any small change in capacity can be in the arrangement can be easily calibrated.

The capacitance C_ of the pressure-responsive capacitor and the capacitance C _{R of} the reference capacitor are preferably chosen so that they are the same at the lowest pressure to be measured. This can be achieved by making the area of the conductive plates 9 and 11 equal to the area of the conductive plates 19i i

and 21 and that between the conductive plates 9 and ' 11 or the conductive plates 19 and 21 an equal distance on the right is obtained. Since the capacitance C of a capacitor is itself , according to the following equation

ί
i
C = K ^ (3)

_5_

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changes, where A is the area of the plates,

D is the distance between the plates, and K is a constant of proportionality,

the distance between the capacitor plates and their area can be changed according to the above equation so that the value of the capacitances C_ and C _n is retained.

The change in the distance between the capacitor plates resulting from the thermal expansion of the shell-shaped parts 3 and 5 when the temperature changes, influences the two capacitances C _a and C _n essentially to the same extent. This means that in the output ratio corresponding to the capacities

signal changes resulting from a thermal off.

elongation would result.

The conductive plates 9 and 11 are circular in shape and are arranged substantially in the middle of the opposing inner surfaces of the shell-shaped parts 3 and 5. The conductive plates 19 and 21 have an annular shape and are on the edges of opposite inner surfaces of the cup-shaped Parts 3 and S arranged. Pressure changes exerted on the housing 1 from the outside lead to a mutual pressure change in this arrangement Moving the conductive plates 9 and 11 and outputting a signal corresponding to the pressure applied, during the conductive plates 19 and 21 are arranged essentially stationary, are not noticeably influenced by changes in pressure and form a reference capacitor. The deflection of the pressure-sensitive circular sections of the cup-shaped

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- 6 parts 3 and 5 is given by the following equation:

3W (m ² - 1 16TTEm ² t ³

1 (a ² - r ² ) ²
2

(D

where W = the external pressure,

m = the reciprocal of the Poisson's number,

a = the disk radius (up to the inside edge of the seal),

r = the radial distance of the point at which the deflection y is observed from the axis of the pressure sensor,

t = the thickness of the disc,

E = the modulus of elasticity.

The maximum deflection of the shell-shaped parts 3 and 5 is at its center point (r = 0). The ratio of the displacement at any point r and the displacement at the midpoint is

2 2 2
_{Y / y} = I (a ² - r ² ) ² (2,

nax a

about the coupling of an interference capacitance in the electrical To avoid measurements, a thin metallic layer can be provided on the outer surfaces of the housing 1, which, as described in US Pat. No. 3,715,638, represents an electrostatic shield.

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As FIG. 3 shows, the reference capacitor formed by the conductive plates 19 and 21 is connected to the input of an amplifier 35. The pressure-responsive capacitor formed by the conductive plates 9 and 11 is connected in a feedback line leading back from the output of the amplifier 35 to its input. The capacitors are connected to voltage via a transformer 25, which has primary windings 27 and 29 connected to an alternating voltage source and secondary windings 31 and 33 inductively coupled thereto, which are connected to the capacitors formed by plates 9 and 11 or 9 and 21. At the output of amplifier 35, the circuit supplies an output signal V which _{is proportional to the quotient of the capacitance C q} of the pressure-responsive capacitor formed by plates 9 and 11 and the capacitance C of the reference capacitor formed by plates 19 and 21. Since the capacitance Cg changes with pressure and the capacitance C _{n has} a fixed value, the output signal V is a function of the pressure. Because the reference capacitor and the pressure-responsive capacitor are formed as molded parts of the pressure transducer, both capacitors are influenced in the same way by temperature changes. Changes in temperature therefore produce little or no error in the output signal V indicating the pressure. That '' both capacitors are contained in a single device, is also advantageous for economic reasons.

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Claims (7)

_ g _. Patent attorneys Dipl. Ing. H. Hauck Dipl. Phys. W. Schmitz Dipl. Ing. E, Graaifs Dipl. Ing. W. Wehnert The Bendix Corporation DiP '· Phys. W. Carstens. 8 Munich 2 Executive Offices Mozartstr. 23 Bendix Center December 13, 1974 Southfield, Mich. 48075, U.S. Attorney File M-3329 Claims 1. Pressure transducer in which a pressure-responsive capacitor and a reference capacitor is used, characterized in that it has a hollow housing (1) which has two spaced apart, facing walls (3,5) made of insulating material, which through insulating ^ma-: TERIAL (6) are joined together at their edges, that the remote from the edge regions of the walls (3,5) are deflected when the pressure changes, while the two edges of the walls not (3,5) portions are substantially located the end-; are steerable, and that the electrodes (9, 11) of the on pressure; Speaking capacitor are arranged on the facing surfaces of the deflectable areas, while the electrodes (19,21) of the reference capacitor on the opposing surfaces of the non-deflectable Areas are arranged. 2. Pressure transducer according to claim 1, characterized in that it has conductors (13, 15) for connecting the capacitors with a 50982870534 ~ ⁹ ~ - Has 9 outer circuit. 3. Pressure transducer according to claim 1 or 2, characterized in that the hollow housing (1) is made of quartz. 4. Pressure transducer according to one of claims 1 to 3, characterized in that that the hollow housing (1) is circular in shape comprises, and that the spaced apart, mutually opposite walls (3,5) in the circumferential direction at their edges are joined together. 5. Pressure transducer according to one of claims 1 to 4, characterized in that that the hollow housing (1) represents a vacuum chamber. 6. Pressure transducer according to one of claims 1 to 5, characterized in that that the electrodes (9,11) of the pressure responsive capacitor are circular and in the middle are arranged, while the electrodes (19,21) of the reference capacitor are circular and are arranged on the edge. 7. Pressure transducer according to one of claims l'.to 6, characterized in that that the pressure-responsive capacitor and the reference capacitor have the same capacitance. 6098-20 / 0534 ^Λ0 Blank page